1. Field of the Invention
The present invention relates to a driving device and a driving circuit for a vibration actuator, and more particularly, to a driving device and a driving circuit for a vibration actuator which are configured such that an undesired vibration can be suppressed and a stable rotation of the vibration actuator can be obtained.
2. Description of the Related Art
An ultrasonic motor (vibration wave driving device) as a type of vibration actuator is mounted in a copier, an interchangeable camera lens, or the like. Along with the recent enhancement in device performance and reduction in power consumption, demands have been made for higher accuracy of rotation and higher efficiency. The feature of the ultrasonic motor is lower-velocity and higher-torque driving than an electromagnetic motor as typified by a DC brushless motor, but there still exists a problem to meet the above-mentioned requirements. For example, depending on a contact state between a stator formed of an elastic member and a piezoelectric element and a rotor formed of a moving member, the behavior may become unstable during rotation to cause an undesired vibration, which is responsible for noise, velocity fluctuations, or uneven wear.
Countermeasures for noise, velocity fluctuations, and uneven wear depending on the contact state are proposed in Japanese Patent Application Laid-Open No. 2005-223994 as a method of superimposing an aperiodic signal on an AC voltage for motor driving. Specifically, the width of a pulse signal for generating the AC voltage to be applied to the vibration actuator is changed in an aperiodic manner.
Otherwise, Japanese Patent Application Laid-Open No. 2003-153559 proposes a method of reducing a distortion factor of the AC voltage for motor driving. Specifically, Japanese Patent Application Laid-Open No. 2003-153559 proposes the concept that a driving sine wave is subjected to frequency variable pulse width modulation (PWM) to have a fixed phase relationship therebetween and a low-distortion sine wave is thereby obtained.